Wafer cleaning in modern VSLI semiconductor processing presents numerous engineering dilemmas. One important issue involves removal of contamination before, during, and after fabrication steps. New processing methods are being developed to work around the deleterious effects (e.g. low-k materials often require less harsh cleaning regimens) of traditional plasma photoresist stripping and cleaning processes. Consequently, non-plasma methods for removing photoresist, residue and contaminants from semiconductor substrates are being developed.
Amongst these new methods, high-pressure processes that employ local densification of a process fluid on the substrate hold promise. Densified fluids are good solvents for contaminants and residues resulting from semiconductor fabrication. Some of these processes, especially those conducted at supercritical pressures, employ additives to increase the solvating power of the process fluid itself. Other additives are used to remove specific contaminants such as polymers, organics, metals, and the like.
Although supercritical fluids are finding acceptance in wafer cleaning regimens, they present many engineering challenges. Most existing apparatus and methods lack flexibility and practicality. Conventional methods and apparatus that employ supercritical fluids for cleaning wafers involve batch type processing. Typically a wafer and one or more cleaning agents are placed in a process vessel. The vessel is sealed. The vessel is then charged with a solvent, and the contents of the process vessel are brought to supercritical conditions. Hence, both cleaning agent dissolution and supercritical solution generation are performed in the presence of the wafer. Once the cleaning process is complete, the process vessel is vented and the substrate is removed. Commonly, opening and closing such vessels is labor intensive. For example, many bolts and components must be secured and removed with each process run. Another disadvantage of traditional vessels is that opening and closing for wafer exchange involves moving heavy components. Overcoming these high inertial loads makes wafer exchange in such systems inefficient.
Another problem with conventional batch type cleaning processes is that they do not allow for easy adjustment in certain process conditions during the course of cleaning. For example, a particular cleaning regimen may call for sequential exposure of a wafer to multiple cleaning agents. This is often necessary when the cleaning agents are hard to dissolve or to keep in solution. In other cases, mixtures of chemical additives for removal of specific contaminants may be used in sequence to perform a cleaning process without removing the substrate from the vessel. In these cases, batch systems are inappropriate because they do not allow easy replacement of one cleaning solution with another in the process vessel, while maintaining supercritical conditions.
Supercritical fluid processes typically require a large volume of supercritical fluid. After processing, oftentimes the processing fluid is vented to a non-recoverable waste stream. This ultimately is bad for the environment and costly. A system that minimizes the amount of supercritical solvents used as well as recycles at least a portion of the solvents is desirable.
What is therefore needed are improved apparatus and methods for cleaning wafers with supercritical fluids. Preferably the apparatus and methods provide flexibility in supercritical fluid cleaning regimens.